1. Field of the Invention
The present invention relates generally to batteries, and more specifically to rechargeable smart batteries that have electronics for predicting the remaining life and recharge time of the battery based on battery-specific characteristics.
2. Related Art
Rechargeable batteries are used in many of today's portable electronic devices, such as computers, camcorders, and cellular phones. Typically, these electronic devices are also capable of utilizing AC power. Battery power is utilized when AC power is not convenient or is not available.
It is often important to provide accurate information regarding the remaining capacity of the battery. Some batteries provide a "fuel gauge" that gives an indication of the charge level of the battery. For example, U.S. Pat. No. 5,315,228 (the '228 patent) describes a rechargeable battery that measures battery discharge current and estimates battery self-discharge to predict the remaining capacity of the battery (i.e., how full the "tank" is to continue the analogy of the "fuel gauge"). Self-discharge refers to a loss in battery capacity that occurs even when the battery is not supplying any discharge current to a load. The '228 patent describes self-discharge as being estimated from experimental observations of battery self-discharge. For example, in the '228 patent a fully charged Ni--MH (nickel metal hydride) battery is estimated to self-discharge at a rate of approximately 6% the first 6 hours and a Ni--Cd (nickel cadmium) battery is estimated to self-discharge at a rate of 3% the first six hours. Both batteries are estimated to self-discharge at rates of about 1.5% the second six hour period, about 0.78% the third and fourth six hour periods, and approximately 0.39% for each subsequent six hour period. For partially discharged batteries, self-discharge is estimated to be about 0.39% for each six hour period. (See column 12, line 59 --column 14, line 4.)
However, this approach for predicting battery capacity does not account for the dependence of self-discharge or battery capacity on environmental conditions, such as battery temperature. FIGS. 1A and 1B illustrate typical self-discharge characteristics as a function of temperature for rechargeable batteries. In certain situations, the '228 patent's method for estimating self-discharge can produce unacceptable errors in the "fitel gauge" remaining capacity indication. Under high temperature conditions, the self-discharge can be much greater than the estimated values. For example, one scenario could include a video camcorder and battery being stored in a car on series of hot, sunny days, causing the battery's "fitel gauge" to erroneously indicate more than enough charge to film a 20-minute wedding ceremony. The scenario concludes with an irate father when his camcorder shuts off 10 minutes into the ceremony, without a spare battery.
To provide warning of low battery conditions, some computer systems provide a run-time alarm that indicates when the battery has less than a fixed amount of time remaining at the present discharge rate. In other words, the alarm is triggered if:
(estimated battery capacity/present discharge rate)&lt; fixed alarm time. The above-described "fuel gauge" is used to indicate battery capacity for this run-time alarm. This type of run-time alarm has several drawbacks in computer systems. First, the present discharge rate does not adequately reflect the dynamic discharge rates in the computer. For example, in today's laptop and notebook computers, power consumption varies dynamically, on the order of several hundred milliamps (mA). This variation is due to power management systems that turn on and off the hard-disk, LCD screen backlight, CPU, etc., under various conditions to save power. Second, the battery capacity provided by the above "fuel gauge" is inaccurate because the dependence of self-discharge on environmental factors, such as temperature, humidity, air pressure, is not considered. Third, the alarm value is fixed. This removes flexibility from the power management system for adjusting the alarm value to the varying power conditions in the system.
Another drawback of today's rechargeable batteries is they do not provide the systems they power with an indication of whether there is enough power remaining to perform a given task, i.e., the power availability of the battery. Although the above "fuel gauge" provides some indication of a battery's remaining capacity, it does not provide specific information about whether the battery can provide a specified amount of additional power to perform a task. For example, near total discharge, a laptop computer may need to know whether there is sufficient battery power available to spin-up the hard disk to save a file before saving the state of the machine and powering down. What the computer needs to know is whether the battery is capable of providing the power for the additional task without the battery's terminal voltage dropping below a cut-off value.
Yet another drawback of today's rechargeable batteries is that they do not provide an accurate indication of remaining battery life based on a user specified discharge rate. The prior "fuel gauge" gives an estimate of the amount of remaining charge in the battery based on the present discharge rate, but provides no indication regarding how long the battery will continue to provide power at other discharge rates. The battery does not tell the user, and the user cannot ask the battery, how long the battery will continue to supply power if the discharge rate is varied. In addition, neither the above "fuel gauge" nor run-time alarm account for the effect of environmental conditions or large battery loads on battery capacity.
Charging is another key aspect of rechargeable batteries. Various methods for charging batteries are known, such as quick charging and trickle charging. With respect to recharge time, the general goal is to charge the battery as quickly as possible without damaging the battery. Charging may cause the battery to heat up. Overheating during charging may damage the battery. For some batteries, such as Ni--Cd, one way of avoiding battery damage during charging is to monitor battery temperature and switch from quick charging to trickle charging if the temperature exceeds a safe level. Such efforts can prevent damage to the battery, but do not necessarily optimize the charging of the battery. For example, minimizing charge time while avoiding destructive conditions may optimize recharge time at the expense of reducing the total number of charge/discharge cycles the battery can provide. Thus, the overall useful life of the battery would be reduced.
Typical battery chargers are designed for use with a specific type of battery. For example, a charger may be designed specifically for charging Ni--Cd batteries, which are charged with a specific construct current, voltage limit, and end-of-charge criteria. However, ongoing improvements to Ni--Cd batteries, such as changes in battery chemistry or cell design, may require that newer Ni--Cd batteries be optimally charged at a different constant current and/or different voltage limit, not provided by the original charger. In addition, charging conditions vary with battery type. For example Ni--MH and Ni--Cd batteries are typically charged at a constant current with a voltage limit. Some lithium-ion and lead-acid batteries are charged at a constant voltage with a current limit. Today's battery chargers are not capable of dynamically adapting to meet the various charging needs of different types of batteries, different battery chemistries, and different battery cell designs. Nor are today's battery chargers capable of adapting to meet changing charging needs as battery chemistries and cell designs change over time.
Another drawback of present battery charging techniques is that no accurate indication is provided as to how long it will take to fully charge the battery from its present capacity. For example, it would be advantageous to know how long it will take to charge a battery that is presently at half capacity.
In summary, rechargeable batteries, such as those used in today's electronic equipment such as laptop computer systems, cellular telephones and video cameras, presently pose a number of problems from both the user's and the equipment's perspective. First, they represent an unpredictable source of power. Typically a user has little advance knowledge that their battery is about to run out or how much operating time is left. Second, equipment powered by the battery can not determine if the battery, in its present state, is capable of supplying adequate power for an additional load (such as spinning up a hard disk). Third, battery chargers must be individually tailored for use with a specific battery chemistry and cell design and may cause damage if used on another battery with a different chemistry or cell design.
Therefore, a smart battery that can predict the remaining life and recharge time of the battery based on battery-specific characteristics is needed.